A traditional electric guitar pickup utilizes magnets and a wire coil to produce sound. It also requires the guitar strings to be made of a ferro-metal. When the ferro-metal strings of the guitar are strummed within the magnetic field produced by the fixed magnets of the pickup, a time-varying voltage is induced in the coil. This time-varying voltage can then be amplified to produce sound. The voltage represents the speed of an instrument string as it vibrates. While this configuration is sufficient to produce sound, it includes limitations with respect to accurately representing the string vibrations, and does not provide the musician with much control of the sound. Furthermore, magnetic pickups can be susceptible to interference from other magnetic or electronic sources, which can diminish sound quality.
In addition to magnetic guitar pickups, optical pickups have been developed. Optical pickups utilize a light field to detect the actual position of the string, thereby enabling more precise play. However, known optical pickups are only offered on custom guitars and must be installed by a manufacturer. Generally speaking, current optical pickups use a trans-illumination configuration. They employ a light source on one side of an instrument string and a sensor diametrically opposite to the light source, creating a shadow of the string on the sensor. The position of the shadow, or of its edge, can be monitored by the sensor and converted into a voltage signal which varies with the motion of the string. This configuration is susceptible to problems with ambient light and typically requires components to be mounted between the strings. It may also have a limited sensing range, allowing it only to be used where the string displacement is very small, and may require “recalibration” when strings are changed. These optical pickups are built into the bridge of the instrument (where the strings are fixed at the tail of the instrument body) and are covered to prevent entry of interfering light. Therefore, if a musician wishes to employ such an optical pickup, he must purchase a new instrument. Not only does this place an economic burden on the musician, but he must replace his current instrument which, apart from the pickup, may be more desirable than the one equipped with the optical pickup.
What is desired is an optical pickup apparatus that can enable precise play and enable sound enhancement and adjustment. Furthermore, what is desired is an optical pickup apparatus that can be installed on an existing instrument.